Furnaces, dryers, roasters, and other heat transfer devices have been used in the processing industries for many decades. Some of the more common furnace designs are rotary kilns, multiple-hearth roasters, and fluid-bed roasters. Direct heating for these furnace designs is common, but other processing devices also utilize indirect heat or energy transfer for heating the material being processed in the devices.
Heat transfer devices which circulate preheated liquids or vapors in an indirect heat transfer configuration typically comprise a jacketed trough into which single or multiple rotating shafts are inserted. Hollow paddles or screws are attached to the shafts and are the means of conveyance for the feed material through the furnace system. The paddles and screws may be intermeshed to facilitate self cleaning. Heat is transferred to the feed material by the introduction of a preheated medium into the jacket and rotating shaft(s) through a set of rotary joints on each end of the shaft. After passing through the system, the heating medium is returned to a heating device for reheating prior to its return to the heat transfer system. Heating limitations of the feed material in this type of heat transfer device depend on the temperature limitations of the heat transfer medium and the rotary joint seals (which deteriorate with excessive temperature) through which the medium passes. Practical limitations are currently in the 650 to 700 degrees Fahrenheit range.
A multiple-hearth roaster consists of a vertical, refractory-lined metal shell containing tiers or hearths mounted one above each other. Material movement is provided by rabble arms on each hearth attached to a central rotating shaft extending through the center of each hearth from the bottom to the top of the roaster. Material is moved by pitched teeth (material is moved toward the outside on one hearth and toward the center on the next lower hearth) attached to each rabble arm. Material enters the furnace at the top hearth and drops through a hole to the hearth below as the material is rabbled back and forth. Burners may be mounted on all or some of the hearths, and the combustion gas flow is generally from lower-to-upper hearths and countercurrent to the flow of the material being processed. Heat transfer is directly from contact between the combustion gases and the feed material. Disadvantages of a hearth furnace are excessive dusting as the processed material falls from hearth to hearth and large discharge gas volumes requiring treatment since generated process gases are combined with combustion gases.
A fluidized-bed roaster generally consists of a vertical, refractory-line metal shell with single or multiple hearths containing a suspension of the coarser fraction of the feed. In some cases where all process feed exits with the fluidizing gases, an inert bed material such as silica sand is added to the system to act as a heat sink. The fluidization of the coarse feed fraction or inert solids is provided by the flow of air, combustion gases, or other types of gases which enter a plenum chamber and pass upward through a constriction plate having a plurality of orifices and then into the fluidized bed. The fluidizing gases may be preheated, or the heat source may be the combustion of gaseous, liquid, or solid fuels in the bed; or electrical means; or by the exothermic nature of the feed. Material to be processed is injected into or above the fluid bed for direct-contact heat transfer. Processed material is removed from the roaster by overflowing from the lowest fluid bed through a conduit and sealed valves and also by the overhead dust collection system. When inert materials are used for bed material, they are recycled to the system. Major disadvantages of fluid bed roasters are high energy costs, the requirement to handle very large volumes of gases, and a limitation on feed material particle size which generally contain material smaller than one-quarter of an inch.
Rotary kilns are comprised of a horizontally-declined shell rotating on trunnions. Material movement through the shell is provided by rotation of the shell and the decline of the shell from feed to discharge end. Mechanical pitched lifters may also be attached to the inside of the shell to facilitate material movement and mixing. Kilns may be heated either directly or indirectly. Like other furnaces, rotary kilns are usually operated under slightly negative pressure to prevent the escape of process gases (those gases generated during thermal treatment) and particulates to the atmosphere. Direct-fired kilns can often contribute to the contamination of the material being processed because of the direct contact of the flame or combustion gases.
Heat to directly-heated kilns is provided by fuel combustion inside the shell or by the introduction of preheated gases from outside the shell. Burner firing may be in either a concurrent configuration (with the flow of the feed) or in a countercurrent configuration (against the flow of the feed). Combustion and generated process gases are combined when direct-firing configurations are used therefore greatly increasing the volume of gas (above the generated process gases) that require treatment for particulate and often acid-gas removal. Direct-fired kilns are usually refractory-lined to prevent metal shell erosion, heat loss from the system, and to protect the metal shell from over heating causing loss of structural integrity. Operating temperatures may range up to 2000 to 3000 degrees Fahrenheit depending on the refractory thickness, insulating ability, and the temperature of the outer shell.
Indirectly-heated kilns use burners fired to impinge a flame on the outside of the rotating shell. Combustion and process gases are kept separated during operation of an indirectly-fired kiln, and therefore, the process gases requiring subsequent treatment is significantly smaller than for a directly-fired kiln. Indirectly-heated kilns are usually internally unlined and employ insulated combustion chambers surrounding the outside of the shell to promote heat transfer to the kiln and processed material. Indirectly heated kilns generally operate at temperatures significantly lower than a directly fired kiln because of metal shell temperature limitations. Maximum operating temperatures typically do not exceed approximately 1000 to 1500 degrees Fahrenheit, depending on the shell material of construction.
For both types of kilns, fuel efficiency is quite low since the heat is either passing over the top of the material being treated in a direct-fired kiln or impinging on the outside of the shell during an indirect-fired application. Fuel efficiencies, without waste heat recovery devices, are usually in the 30- to 40-percent range.
Indirectly-heated kilns may also employ numerous resistance-type heaters surrounding the outside of, but not attached to, the rotating shell. For larger-size, rotary kiln-type systems heated in this manner, there is often uneven heating or cooling of the kiln's surface which may result in severe warping of the shell. Also, since the heating apparatus is not attached to (but surrounded by) the shell, energy transfer to the processed material inside the shell is still quite inefficient.
Regardless of the type of rotary kiln, a relatively complicated set of seals (single or double) or end plates is generally required on each end of the rotating shell. These seals or end plates, which are nonrotating and the same diameter as the rotating shell diameter, are particularly critical when it is essential to control the composition of the process gases or prevent the discharge of fugitive emissions to the atmosphere. Because the kiln operates under negative pressure, the seals are usually directly purged (or purged between the seals in the case of double seals that are separated from each other) with relatively large quantities of inert gas, such as nitrogen or steam, to prevent the ingress of air or other vapor into the system. These purge gases contaminate the internal process environment, and when combined with the process gases, significantly increase the total volume of the gases through the shell, therefore requiring increased sizing of down stream vapor-handling equipment.
Because the shell is constantly rotating, the use of instrumentation, such as internal temperature- and pressure-measuring devices, in long kilns is often difficult. The sensing probes are usually introduced through the fixed kiln end plates which limits the zones for taking measurements. Further, the tumbling action of the material being processed, caused by the rotation of the shell, often tends to create excessive dusting resulting in excessive particulate loading of the process vapors. This tumbling action also tends to promote material size segregation.
U.S. Pat. No. 4,931,610 to Hughes et al describes a kiln-type apparatus heated by induction. Alternating current energizing a conducting coil wound around, and insulated from, an internal rotating metal shell generates an alternating electromagnetic field that induces a current in the metal shell. Heat is then generated in the metal shell by electrical resistance of the metal. The shell is not connected directly to the power source, and the rotating shell still requires lip seals for containment of internal gases and exclusion of the outside atmosphere.
U.S. Pat. No. 5,144,108 to Passarotto describes a stationary, cylindrical-type apparatus for the conversion of massicot to litharge. The metal shell is surrounded by, and insulated from, a wound coil energized by an alternating current source similar to that described in the Hughes patent. Material is conveyed through the shell by paddles mounted on a rotating shaft extending the length of the shell. Heat is induced in the metal shell and not connected directly to the power source.
U.S. Pat. No. 4,039,794 to Kasper describes an induction-heated system for heating ferromagnetic abrasive shot in a rotating, cylinder-shape apparatus using lip seals for the exclusion of the external atmosphere from the interior of the cylinder. Also U.S. Pat. No. 3,961,150 to Lewis et al describes an induction-heated device for sterilizing sealed, electrically conductive containers.
Impedance heating of liquids and gases conveyed in pipelines has been used in four basic applications: namely, (1) applying heat to increase the fluidity of static, viscous materials so they can be pumped, particularly for oil transportation through pipelines; (2) maintaining temperature (offset heat loss) of transported liquids and gases flowing through a pipe and for freeze protection; (3) heating fluid liquids and gases passing through a pipe; and (4) heating fluid liquids stored in tanks. Representative examples of materials commonly transported through an impedance-heated line include crude oil, fuel oil, tar, paraffin, resin, sulfur, and chocolate, all of which are fluid and flowable at the elevated temperature of the line.
U.S. Pat. No. 3,777,117 to Othmer describes an impedance heating device suitable for heating fluid in long pipelines relying on the "skin effect" of the pipe's inner surface created by the alternating current. Location of the power cable and the use of internal fins inside the pipeline are used to more effectively transfer the heat to the fluid.
Additional impedance-heated devices disclosed in U.S. Pat. Nos. 3,632,975 and 4,578,564 to Ando and Takagi describe an improved impedance-heated pipe for heating fluids. The invention relates to a long heat-generating pipe utilizing the skin effect of alternating current having one or more impedance elements in the circuit in order to offset nonuniformity of current through the pipeline. U.S. Pat. No. 4,110,599 to Offermann describes a method of reducing the heat output of an impedance-heated pipe by making a segment of the pipe of a non-electrically-conducting or non-ferromagnetic material such as aluminum. U.S. Pat. No. 4,408,117 to Yurkanin describes a "skin effect" impedance heating system for tanks or vessels containing a liquid. A pipe (or several pipes) is inserted through the side of a tank wall, and the insulated power cable is run down the center of the pipe to produce the electromagnetic magnetic flux to generate the "skin effect" heating.
This invention has as its purpose to provide for a processing device or furnace for the thermal treatment of materials which are nonflowable at processing temperatures, while eliminating or overcoming many of the disadvantages or limitations of prior art devices.